Conventionally, compounds having cytotoxicity have been used as antitumor agents, and a lot of screenings have been carried out using cytotoxicity as an index. As a result, since most of the conventional antitumor agents affect cancer cells and, at the same time, normal tissues with active cell proliferation, for example, the bone marrow and intestine epithelium, QOL of patients is not sufficiently improved.
Further, under existing circumstances, antitumor agents have come to have a rather beneficial effect on treating leukemia, but are not necessarily effective for solid tumors. Therefore, antitumor agents that are effective for solid tumors and are highly safe have been strongly demanded.
Fermentation products of microorganisms have been screened mainly using cytotoxicity in vitro as an index, in order to use these products as antitumor agents. As a result, many cytotoxic compounds have been discovered. However, most of the compounds have been confirmed to show cytotoxic activities only in vitro, and few compounds have been found to have an antitumor activities in vivo. Furthermore, very few compounds exhibit efficacy against solid cancers.